Three-speed transfer case

ABSTRACT

A transfer case having an input shaft, an output shaft, and a planetary gearset connected therebetween. The planetary gearset includes a first sun gear, a second sun gear, a carrier coupled for rotation with the input shaft, a ring gear coupled for rotation with the output shaft, and meshed pairs of first and second pinions rotatably supported on the carrier with each first pinion meshed with the first sun gear and each second pinion meshed with the second sun gear and the ring gear. The transfer case further includes a powershift clutch assembly comprised of a first range clutch located between the carrier and the ring gear, a second range clutch located between the first sun gear and a stationary member, and a third range clutch located between the second sun gear and the stationary member. Actuation of the clutches is controlled by a hydraulic control system for shifting each of the clutches between actuated and non-actuated modes to establish three different speed ratio drive connections between the input and output shafts.

FIELD OF THE INVENTION

The present invention relates generally to transfer cases for use infour-wheel drive motor vehicles. In particular, the present invention isdirected to a three-speed powershift transfer case.

BACKGROUND OF THE INVENTION

Due to increased consumer demand for four-wheel drive vehicles, aplethora of different power transfer systems are currently utilized fordirecting power (i.e., drive torque) to all four wheels of the vehicle.In many applications, a transfer case is used to transfer power from thepowertrain to the front and rear drivelines. Moreover, many moderntransfer cases are equipped with a two-speed gear reduction unit forestablishing high-range and low-range drive modes to accommodatediffering road conditions and off-road terrains. Typically, thehigh-range drive mode is a direct speed ratio for use during most normaldriving conditions while the low-range drive mode is an underdrive ratiouseful during low-speed off-road conditions.

The two-speed gear reduction unit used in most transfer cases is eithera layshaft arrangement or a planetary gearset with one or more clutchesthat are selectively actuated for making the drive connections. As aconvenience feature, many gear reduction units now include a system forpermitting such “range” shifting to be accomplished without firststopping the vehicle. In some instances, synchronized dog clutches areused to permit “on-the-move” shifting between the high-range andlow-range drive modes. Examples of such synchronized range shift systemsfor transfer cases are disclosed in commonly-owned U.S. Pat. Nos.5,346,442; 5,655,986; 5,702,321; 5,836,847; and 5,902,205.Alternatively, it is known to employ a powershift clutch arrangement intwo-speed transfer cases for shifting on-the-move between the high-rangeand low-range drive modes. In this regard, U.S. Pat. Nos. 5,443,429;5,688,202; and 5,700,222 disclose powershift-type range shift systems intransfer cases.

In most four-wheel drive vehicles equipped with a transfer case, thetransmission output shaft is coupled to the transfer case input shaft.However, in some applications, an add-on gearbox, commonly referred toas a compounder assembly, is installed between the transmission outputshaft and the transfer case input shaft for providing an additional gearratio, such as an “overdrive” ratio. An example of an add-on overdrivetransmission assembly is shown in commonly-owned U.S. Pat. No.4,798,103. Due to the long leadtime and cost associated with developingnew multi-speed transmissions, the use of a compounder assembly inconjunction with an existing transmission for providing an additionalgear ratio has found commercial success. One design compromise, however,is the additional length required in the vehicle's powertrain to acceptinstallation of the compounder assembly.

From the foregoing, it is apparent that transfer cases equipped withtwo-speed gear reduction units have been successfully integrated intofour-wheel drive vehicles. However, a need still exists to developalternative gear reduction units that further advance the technology inthe field of four-wheel drive transfer cases.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide athree-speed transfer case for use in four-wheel drive vehicles.

As a related object, the transfer case of the present invention includesa powershift clutch assembly in association with a planetary gearset forestablishing three distinct speed ratio drive connections between aninput shaft and an output shaft.

According to another object, the present invention integrates thefunction of an add-on compounder and a two-speed gear reduction unitinto a transfer case for use in four-wheel drive vehicles. In thisregard, the three-speed transfer case of the present invention isoperable to establish a first drive connection, a second driveconnection, and a third drive connection between the input shaft and theoutput shaft.

According to the present invention, the transfer case includes an inputshaft, an output shaft, and a planetary gearset connected therebetween.The gearset includes a first sun gear, a second sun gear, a carriercoupled for rotation with the input shaft, meshed pairs of first andsecond pinions rotatably supported on the carrier with each first pinionmeshed with the first sun gear and each second pinion meshed with thesecond sun gear, and a ring gear coupled for rotation with the outputshaft and which is meshed with the first or second pinions. The transfercase further includes a powershift clutch assembly comprised of a firstrange clutch located between the carrier and the ring gear, a secondrange clutch located between the first sun gear and a stationary member,and a third range clutch located between the second sun gear and thestationary member. Actuation of the range clutches is controlled by ahydraulic control system for shifting each range clutch between actuatedand non-actuated modes to establish the three different speed ratiodrive connections. The three speed ratios include a direct drive speedratio, an underdrive speed ratio and an overdrive speed ratio.

In accordance with an alternative embodiment, the planetary gearsetincludes a compound pinion gear rotatably supported on the carrier andhaving a first pinion meshed with the first sun gear and a second pinionmeshed with the second sun gear and the ring gear. Actuation of thethree range clutches establishes three speed ratios including a directspeed ratio and two different overdrive speed ratios.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from a reading of thefollowing description in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic illustration of a four-wheel drive motor vehicleequipped with the three-speed transfer case of the present invention;

FIG. 2 is a schematic illustration of a three-speed transfer caseaccording to the present invention;

FIG. 3 is a partial sectional view of a three-speed transfer case, basedon the schematic version shown in FIG. 2, showing the planetary gearsetand powershift clutch assembly in greater detail;

FIG. 4 is a schematic of a hydraulic control system used for controllingactuation of the powershift clutch assembly;

FIG. 5 is a chart listing the various drive modes available with thethree-speed transfer case of the present invention;

FIG. 6 is a schematic illustration of the three-speed transfer caseoptionally equipped with a different planetary gearset;

FIG. 7 is a chart listing the drive modes available with the three-speedtransfer case of FIG. 6; and

FIG. 8 is another schematic illustration of the three-speed transfercase optionally equipped with a compound pinion planetary gearset.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, a drivetrain 10 for a four-wheel drivevehicle is shown to include a powertrain 12 for supplying power (i.e.,drive torque) to front driveline 14 and rear driveline 16. Powertrain 12includes an engine 18, a multi-speed transmission 20, and a transfercase 22. Transfer case 22 has an input shaft 24 driven by an outputshaft of transmission 20, a rear output shaft 26, a planetary gearset28, and a powershift clutch assembly 30 which can be selective actuatedto engage gearset 28 for establishing three distinct drive connectionsbetween input shaft 24 and rear output shaft 26. Transfer case 22 alsoincludes a front output shaft 32 and a transfer mechanism 34 forsupplying drive torque from rear output shaft 26 to front output shaft32.

Front driveline 14 includes a front propshaft 36 interconnecting frontoutput shaft 32 to a differential 38 of a front axle assembly 40 fordriving front wheels 42. Similarly, rear driveline 16 includes a rearpropshaft 44 interconnecting rear output shaft 26 to a differential 46of a rear axle assembly 48 for driving rear wheels 50. As furtherillustrated in FIG. 1, a controller 52 receives sensor signals fromvehicle sensors 54 and a mode signal from a mode select mechanism 56 foruse in controlling actuation of a hydraulic control system 58 inresponse to a particular drive mode selected by the vehicle operator. Aswill be detailed, hydraulic control system 58 functions to controlactuation of three range clutches associated with powershift clutchassembly 30 and a transfer clutch associated with transfer mechanism 34.

With particular reference to FIGS. 2 and 3, transfer case 22 includes ahousing 60 which rotatably supports input shaft 24, rear output shaft26, and front output shaft 32 via suitable bearing assemblies. Gearset28 is shown to include a carrier assembly 62 driven by input shaft 24, afirst sun gear 64, a second sun gear 66, a ring gear 68 driving rearoutput shaft 26, a set of first pinions 70 rotatably supported fromcarrier assembly 62 and meshed with first sun gear 64, and a set ofsecond pinions 72 rotatably supported from carrier assembly 62 andmeshed with second sun gear 66 and ring gear 68. First and secondpinions 70 and 72 are arranged in meshed pairs and are retained betweena front carrier ring 74 and a rear carrier ring 76 which are boltedtogether to define carrier assembly 62. As seen, rear carrier ring 76includes a hub segment 98 which is fixed via a splined connection 78 forrotation with input shaft 24 and is axially restrained thereon by snaprings 80. As noted, powershift clutch assembly 30 includes three rangeclutches which include a first range clutch 82 located between ring gear68 and carrier assembly 62, a second range clutch 84 located betweenhousing 60 and first sun gear 64, and a third range clutch 86 locatedbetween housing 60 and second sun gear 66. In addition, transfermechanism 34 includes a transfer assembly 87 and a transfer clutch 88located between rear output shaft 26 and transfer assembly 87 fortransferring drive torque from rear output shaft 26 to front outputshaft 32. Transfer assembly 87 includes a drive sprocket 90 supportedfor rotation relative to rear output shaft 26, a driven sprocket 92fixed for rotation with front output shaft 32, and a power chain 94meshed with drive sprocket 90 and driven sprocket 92.

As best seen from FIG. 3, first range clutch 82 is a multi-plate wetclutch assembly that is spring-loaded to normally function in a “locked”mode so as to couple ring gear 68 for common rotation with carrierassembly 62. First range clutch 82 includes a clutch pack 96 mountedbetween a hub segment 98 of rear carrier ring 74 and a drum housing 100fixed for rotation with rear output shaft 26. Clutch pack 96 iscomprised of a set of inner clutch plates that are splined to hubsegment 98 and which are alternately interleaved with a set of outerclutch plates that are splined to drum housing 100. Drum housing 100includes an end plate 102 that is fixed via a splined connection 104 torear output shaft 26, and a cylindrical outer drum 106 welded to endplate 102. As seen, ring gear 68 is formed on a raised end segment 108of outer drum 106 which terminates with a radial flange 110. First rangeclutch 82 also includes a spring-loaded piston assembly 112 comprised ofa piston 114, a piston sleeve 116 fixed (i.e., welded) to piston 114, apressure plate 118 fixed (i.e., welded) to piston sleeve 116, and apiston drum 120 also fixed to piston sleeve 116. Piston 114 is retainedin a pressure chamber 122 which is hydraulically connected to hydrauliccontrol system 58 via a first flow pathway 124. First flow pathway 124includes an axial slotted channel 126 a formed in rear output shaft 26,a radial port 126 b formed through end plate 102, and an axial bore 126c formed in a bulkhead 60 a of housing 60. Slotted channel 126 a isaligned with missing spline teeth associated with splined connection 104to permit fluid flow to pressure chamber 122. Suitable O-ring seals areshown for providing fluid-tight sealed junctions between end plate 102and corresponding surfaces of rear output shaft 26 and bulkhead 60 a.

With continued reference to FIG. 3, piston drum 120 is shown to have anend segment defining lugs 128 which extend through slots 130 formed inend segment 108 of outer drum 106. Piston drum 120 extends over andencloses radial flange 110 of outer drum 106 so as to define an annularspring chamber 132 therewith. A plurality of coil springs 134 aremounted in spring chamber 132 between spring flange 110 and lugs 128 tonormally bias piston assembly 112 in a rearward direction (i.e., to theright in FIG. 3) to an engaged position for causing pressure plate 118to exert a maximum clamping load on clutch pack 96, thereby couplingring gear 68 for common rotation with carrier assembly 62 andestablishing the “locked” mode of first range clutch 82. As seen, areaction plate 136 is retained against outer drum 106 such that clutchpack 96 is located between reaction plate 136 and pressure plate 118.Due to lugs 128 being retained in slots 130, piston assembly 112 rotateswith drum housing 100 and is biased by springs 134 to its engagedposition. Upper and lower O-ring seals are shown for maintaining afluid-tight sliding seal between piston 114 and corresponding wallsurfaces of drum housing 100 and rear output shaft 26. As will bedetailed, the supply of high pressure fluid from hydraulic controlsystem 58 to pressure chamber 122 via first flow pathway 124 causespiston assembly 112 to move axially in a forward direction from itsengaged position to a released position. With piston assembly 112 in itsreleased position, pressure plate 118 disengages clutch pack 96 topermit relative rotation between carrier assembly 62 and ring gear 68such that first range clutch 82 is considered to be functioning in a“released” mode.

Second range clutch 84 is shown as a multi-plate wet clutch assemblyincluding a clutch pack 140 mounted between housing 60 and a clutch drum142 that is fixed to first sun gear 64. Clutch pack 140 includes a setof outer clutch plates that are splined to housing 60 and which arealternately interleaved with a set of inner clutch plates that aresplined to a cylindrical hub segment 144 of clutch drum 142. A reactionplate 146 is also splined to housing 60 and is axially restrained by asnap ring 150. In addition, an apply plate 152 is splined to housing 60such that clutch pack 140 is retained between apply plate 152 andreaction plate 146. Second range clutch 84 also includes a piston 154disposed in an annular pressure chamber 156 formed in housing 60 andwhich is hydraulically connected to hydraulic control system 58 via asecond flow pathway 158. Piston 154 is shown retracted in a forwarddirection to a released position such that apply plate 152 is releasedfrom clamped engagement with clutch pack 140 so as to permitunrestricted rotation of first sun gear 64 relative to housing 60. Withpiston 154 located in its released position, second range clutch 84 isconsidered to be functioning in a “released” mode. However, delivery ofhigh pressure fluid to pressure chamber 156 via second flow pathway 158causes piston 154 to move in a rearward direction from its releasedposition to an engaged position for causing apply plate 152 to exert aclamping load on clutch pack 140, thereby braking rotation of first sungear 64. With piston 154 in its engaged position, second range clutch 84is defined as operating in a “locked” mode. Upper and lower O-ring sealsare provided to maintain a fluid-tight seal between piston 154 andcorresponding wall surfaces of housing 60 to define pressure chamber156.

With continued reference to FIGS. 2 and 3, third range clutch 86 isshown as a multi-plate wet clutch assembly including a clutch pack 162mounted between a bulkhead segment 60 b of housing 60 and a clutch drum164 that is fixed to second sun gear 66. Clutch pack 162 includes a setof outer clutch plates splined to a drum segment 166 of clutch drum 164and which are alternately interleaved with a set of inner clutch platessplined to bulkhead 60 b. A reaction plate 168 is splined to bulkhead 60b and is axially restrained by a snap ring 170. As seen, clutch pack 162is located between reaction plate 168 and an apply plate 172 that isalso splined to bulkhead 60 b. Third range clutch 86 also includes apiston 174 disposed in a pressure chamber 176 defined between bulkhead60 b and piston 154 of second range clutch 84, with suitable O-ringseals providing a fluid-tight seal relative thereto. Pressure chamber176 is hydraulically connected to hydraulic control system 58 via athird flow pathway 178 formed in bulkhead 60 b. Piston 174 is shown in areleased position such that apply plate 172 is disengaged from clutchpack 162 so as to permit unrestricted rotation of second sun gear 66relative to housing 60. With piston 174 located in its releasedposition, third range clutch 86 is considered to be operating in a“released” mode. Delivery of high pressure fluid to pressure chamber 176via third flow pathway 178 causes piston 174 to move from its releasedposition to an engaged position for causing apply plate 172 to exert aclamping load on clutch pack 162, thereby braking rotation of second sungear 66. With piston 174 in its engaged position, third range clutch 86is considered to be operating in a “locked” mode.

Transfer clutch 88 is a multi-plate wet clutch assembly that isspring-loaded to normally function in a “2WD” mode with all drive torquetransmitted to rear output shaft 26. Transfer clutch 88 includes aclutch drum 190 fixed to drive sprocket 90, a clutch pack 192 mountedbetween a cylindrical hub segment 194 of clutch drum 190 and outer drum106 of drum housing 100, and a spring-biased piston assembly 196. Clutchpack 192 includes a set of outer clutch plates that are splined to hubsegment 194 of clutch drum 190 and which are alternately interleavedwith a set of inner clutch plates that are splined to outer drum 106. Areaction plate 200 is also splined to hub segment 194 of clutch drum 190and is retained thereon by a snap ring 202. In addition, an apply plate204 is splined to hub segment 194 of clutch drum 190. Piston assembly196 includes a piston 206 disposed in a pressure chamber 208 formed byclutch drum 190 and which is hydraulically connected to hydrauliccontrol system 58 via a fourth flow pathway 210. Pathway 210 includesone or more radial bores 212 a formed through sprocket 90 and a channel212 b formed in bulkhead 60 b with appropriate O-ring seals providing afluid seal between sprocket 90 and bulkhead 60 a. In addition, suitableO-ring seals provide a fluid-tight seal between piston 206 and wallsurfaces of clutch drum 190.

A plurality of coil springs 214 are retained between piston 206 and aspring retainer plate 216 that is secured via a snap ring 218 to clutchdrum 190. Springs 214 normally bias piston 206 in a rearward directionto a released position such that apply plate 204 exerts a predeterminedminimum clutch load on clutch pack 192 to permit relative rotationbetween drum housing 100 and drive sprocket 90. Delivery of pressurizedfluid to pressure chamber 208 via fourth flow pathway 210 causes piston206 to move, in opposition to the biasing of springs 214, from itsreleased position toward an engaged position. Such movement of piston206 causes apply plate 204 to exert a corresponding clamping load onclutch pack 192, thereby transferring drive torque through transferassembly 87 to front output shaft 32 for establishing a “4WD” mode.Additionally, movement of piston 206 to a fully-engaged position resultsin a maximum clutch load being exerted on clutch pack 192 such thattransfer clutch 88 rigidly couples drive sprocket 90 for common rotationwith rear output shaft 26, whereby transfer clutch 88 is considered tobe operating in a “4WD Lock” mode. Moreover, based on the magnitude ofthe fluid pressure supplied to pressure chamber 208, the axial positionof piston 206 can be controlled between its released position and itsfully-engaged position. Thus, the fluid pressure supplied by hydrauliccontrol system 58 can be controlled to modulate the rear-to-front torquetransfer ratio across transfer clutch 88 in a range from 100%-0% to50%-50% so as to provide a “4WD ADAPTIVE” mode of operation with optimaltorque transfer control based on road conditions and sensed vehicleoperating conditions. An example of a control system applicable tocontrol such adaptive or on-demand operation of transfer clutch 88 canbe found in commonly-owned U.S. Pat. No. 5,688,202. However, since thenovelty of the present invention resides in the three-speed powershiftclutch and gearset arrangement, those skilled in the art will appreciatethat other actively-controlled (i.e., electromagnetic clutches,cam-actuated clutches, etc.), passively-controlled (i.e., viscouscouplings, geared traction couplings, hydromechanical couplings, etc.)can be substituted for the particular hydraulically-actuated transferclutch 88 disclosed. In addition, it is contemplated that drum housing100 could act as the driven input member of an interaxle differentialwhich connects the front and rear output shafts in a full-time transfercase instead of being coupled to rear output shaft 26. In such afull-time arrangement, transfer clutch 88 would be used to regulatespeed differentiation and torque biasing across the interaxledifferential.

In operation, transfer case 22 is capable of being powershifted forestablishing three distinct drive connections between input shaft 24 andrear output shaft 26. In particular, a first drive connection isestablished with piston 114 of first range clutch 82 in its engagedposition, piston 154 of second range clutch 84 in its released position,and piston 174 of third range clutch 86 in its released position. Thus,with fluid pressure vented from pressure chambers 122, 156 and 176,clutch pack 96 of first range clutch 82 couples carrier assembly 62 toring gear 68, whereby rear output shaft 26 is driven at the same speedas input shaft 24 to define a direct drive or “high-range” drive mode.Accordingly, the high-range drive mode is established when first rangeclutch 82 is in its locked mode while second range clutch 84 and thirdrange clutch 86 are in their released modes.

When it is desired to shift transfer case 22 into an underdrive or“low-range” drive mode such as, for example, when the vehicle is beingdriven off-road, a mode signal is sent by mode shift mechanism 56 tocontroller 52. Controller 52 then sends control signals to hydrauliccontrol system 58 for establishing a second drive connection betweeninput shaft 24 and rear output shaft 26. The second drive connection isestablished by supplying high pressure fluid to pressure chambers 122and 156 while pressure chamber 176 is vented. The supply of pressurizedfluid to pressure chamber 122 causes piston 114 of first range clutch 82to move from its engaged position to its released position for releasingclutch pack 96, thereby releasing ring gear 68 for rotation relative tocarrier assembly 62. In addition, venting of pressure chamber 176 causespiston 174 of third range clutch 86 to be maintained in, or move to, itsreleased position for releasing clutch pack 162 such that second sungear 66 is free to rotate. Likewise, the supply of pressurized fluid topressure chamber 156 causes piston 154 of second range clutch 84 to movefrom its released position to its engaged position for causing clutchpack 140 to brake first sun gear 64. The geometry of the gear componentsassociated with planetary gearset 28 causes rear output shaft 26 to bedriven at a reduced speed relative to input shaft 24 so as to define thelow-range drive mode. Accordingly, the low-range drive mode isestablished when first range clutch 82 is in its released mode, secondrange clutch 84 is in its locked mode, and third range clutch 86 is inits released mode. To assure against locking of planetary gearset 28, itis understood that first range clutch 82 is shifted into its releasedmode prior to shifting of second range clutch 84 into its locked mode.

When it is desired to shift into an “overdrive” mode such as, forexample, during high-speed highway driving, a mode signal is sent bymode shift mechanism 56 to controller 52. In response to this modesignal, controller 52 causes hydraulic control system 58 to send highpressure fluid to pressure chambers 122 and 176 while pressure chamber156 is vented. This action causes piston 114 of first range clutch 82 tomove from its engaged position to its released position for releasingclutch pack 96. Moreover, venting of pressure chamber 156 causes piston154 of second range clutch 84 to be maintained in, or move to, itsreleased position for releasing clutch pack 140 such that first sun gear64 is free to rotate. In addition, the delivery of high pressure fluidto pressure chamber 176 causes piston 174 of third range clutch 86 tomove from its released position to its engaged position for causingclutch pack 162 to brake second sun gear 66. As such, the third driveconnection is established with rear output shaft 26 driven at anincreased speed relative to input shaft 24, thereby defining theoverdrive mode. Thus, the overdrive mode is established with third rangeclutch 86 in its locked mode while first range clutch 82 and secondrange clutch 84 are in their released modes. Again, clutch pack 96 offirst clutch 82 is released prior to shifting of third range clutch 86into its locked mode.

A Neutral non-driven mode can be established by shifting first rangeclutch 82 into its released mode to release clutch pack 96 while secondrange clutch 84 and third range clutch 86 are shifted into, ormaintained in, their released modes. In this Neutral mode, no drivetorque is transferred from input shaft 24 to rear output shaft 26.

Referring now to FIG. 4, the various components associated withhydraulic control system 58 are schematically shown. Hydraulic controlsystem 58 includes a reservoir or sump 230 containing hydraulic fluid, apump 232, and an accumulator 234. Pump 232 draws fluid from sump 230 andsupplies high pressure fluid to accumulator 234. A firstelectrically-controlled solenoid flow control valve 236 is located in afirst supply line 238 between accumulator 234 and first flow pathway 124for controlling the supply of hydraulic fluid to and from pressurechamber 122 of first range clutch 82. A second electrically-controlledsolenoid flow control valve 240 is located in a second supply line 242between accumulator 234 and second flow pathway 158 for controlling thesupply of hydraulic fluid to and from pressure chamber 156 of secondrange clutch 84. A third electrically-controlled solenoid flow controlvalve 244 is located in a third supply line 246 between accumulator 234and third flow pathway 178 for controlling the supply of hydraulic fluidto and from pressure chamber 176 of third range clutch 86. Finally, afourth electrically-controlled solenoid flow control valve 248 islocated in a fourth supply line 250 between accumulator 234 and fourthflow pathway 210 for controlling the supply of hydraulic fluid to andfrom pressure chamber 208 of transfer clutch 88. In one preferredarrangement, the first, second and third solenoid valves are of theon/off (i.e., two-position) variety while fourth control valve 248 is ofthe variable output type to permit modulation of the fluid pressure inpressure chamber 208 of transfer clutch 88. Controller 52 sendsappropriate electrical control signals to each of the solenoid valves tocontrol actuation thereof which, in turn, controls the supply ofhydraulic fluid to and from each of the pressure chambers. Sensor inputsignals are sent to controller 52 from vehicle sensors 52 and are usedfor controlling adaptive actuation of transfer clutch 88 and if desired,to automatically control the conditions under which powershifts arepermitted. Vehicle sensors 52 is intended to be indicative of sensorsused for detecting or measuring such operating parameters as vehiclespeed, interaxle speed differentiation, transmission gear, brake status,throttle position and other signals used to control driveline systems.Hydraulic pressure generated by pump 232 may be caused by drivenrotation of a rotary component of transfer case 22 or, more preferably,by means of an electric motor 252. While not shown, it will be obviousthat hydraulic fluid vented from the pressure chamber of each of rangeclutches 82, 84 and 86 and transfer clutch 88 is returned to sump 230.

Transfer case 22 is capable of establishing a number of distinctoperative modes. A particular operative mode is established based on theactuated state of first range clutch 82, second range clutch 84, thirdrange clutch 86 and transfer clutch 88. As is known, mode selectmechanism 56 can take the form of any mode selector device under thecontrol of the vehicle operator which can be manipulated to generate amode signal indicative of the specific operative mode selected. Assumingavailability of all of the operative modes, the various actuated stateof each clutch is shown in the table provided in FIG. 5. Obviously,typical vehicular applications would limit the available number ofoperative modes to choose from, but FIG. 5 is exemplary of theflexibility provided by the present invention. In addition, it iscontemplated that shifting between the high-range mode and the overdrivemode may be controlled automatically in coordination with controlledshifting of transmission 20. Thus, transfer case 22 permits selection ofhigh and low speed ranges as well as providing an overdrive ratio thatpermits establishment of an additional forward gear ratio in conjunctionwith transmission 20.

Referring now to FIG. 6, an alternative version of a transfer case 22Ais shown to include a modified planetary gearset 28A having bigger firstpinions 70A meshed with first sun gear 64 and ring gear 68 while smallersecond pinions 72A are meshed with second sun gear 66. Pinions 70A and72A are arranged in meshed pairs and are rotatably supported from planetcarrier 62. In addition, third range clutch 86 is now used to establishthe “low-range” drive mode while second range clutch 84 is used toestablish the “overdrive” mode. Thus, the low-range drive mode isestablished with first range clutch 82 in its released mode, secondrange clutch 84 in its released mode, and third range clutch 86 in itslocked mode. Similarly, the overdrive mode is established with firstrange clutch 82 and third range clutch 86 in their released modes withsecond range clutch 84 in its locked mode. This reversal in clutchactuation and the revised meshing and geometry associated with pinions70A and 72A of gearset 28A permit generation of a lower numeric torqueratio in the low-range drive mode compared to that available withgearset 28 of transfer case 22 shown in FIGS. 2 and 3. The Tableprovided in FIG. 7 shows the clutch actuation states required toestablish the various 2WD and 4WD operative modes at the three speedranges.

Referring now to FIG. 8, a further alternative version of a transfercase 22B is shown to be equipped with a planetary gearset 28B having aset of compound pinion gears 71 comprised of a first pinion 70B fixedto, or integral with, a second pinion 72B. As seen, first pinion 70B ofeach pinion gear 71 is meshed with first sun gear 64 while each secondpinion 72B is meshed with second sun gear 66 and ring gear 68. Compoundpinion gears 71 are rotatably supported from planet carrier 62. Withthis arrangement, the high-range and overdrive modes are established ina manner identical to that previously described in association withtransfer case 22 of FIG. 2. However, actuation of second range clutch 84in coordination with the release of first and third range clutches 82and 86, respectively, does not establish the low-range drive modeprovided by transfer case 22, but rather established a second overdrivemode. In particular, braking of first sun gear 64 causes ring gear 68 torotate at an increased speed relative to carrier 62. In this manner twodistinct overdrive ratios can be established in addition to the direct(high-range) speed ratio.

The foregoing discussion discloses and describes exemplary embodimentsof the present invention. One skilled in the art will readily recognizefrom such discussion, and from the accompanying drawings and claims,that various changes, modifications and variations can be made thereinwithout departing from the true spirit and fair scope of the inventionas defined in the following claims.

What is claimed is:
 1. A transfer case comprising: a housing supporting an input shaft and an output shaft; a planetary gearset including a first sun gear, a second sun gear, a carrier coupled to said input shaft, a first pinion supported on said carrier and meshed with said first sun gear, a second pinion supported on said carrier and meshed with said second sun gear and said first pinion, and a ring gear coupled to said output shaft and meshed with one of said first and second pinions; a first clutch operable in a locked mode to couple said ring gear for common rotation with said carrier and which is further operable in a released mode to permit relative rotation therebetween; a second clutch operable in a released mode to permit rotation of said first sun gear and which is further operable in a locked mode to brake said first sun gear; a third clutch operable in a released mode to permit rotation of said second sun gear and which is further operable in a locked mode to brake said second sun gear; and a control system for controlling actuation of said first, second and third clutches to establish three drive connections between said input shaft and said output shaft, wherein a first drive connection is established with said first clutch in its locked mode and each of said second and third clutches in its released mode, wherein a second drive connection is established with said second clutch in its locked mode and each of said first and third clutches in its released mode, and wherein a third drive connection is established with said third clutch in its locked mode and each of said first and second clutches in its released mode.
 2. The transfer case of claim 1 wherein said first clutch includes a first clutch pack mounted between said carrier and said ring gear, a piston movable between a first position and a second position relative to said first clutch pack, and a biasing mechanism for biasing said piston toward its first piston, said piston is operable in its first position to engage said first clutch pack to couple said ring gear for rotation with said carrier and define said locked mode of said first clutch, and said piston is operable in its second position to release said first clutch pack to permit relative rotation between said ring gear and said carrier and define said released mode of said first clutch.
 3. The transfer case of claim 2 wherein said second clutch includes a second clutch pack mounted between said housing and said first sun gear, and a second piston movable between a first position and a second position relative to said second clutch pack, said second piston is operable in its first position to engage said second clutch pack to brake rotation of said first sun gear and define said locked mode of said second clutch, and said second piston is operable in its second position to release said second clutch pack to permit rotation of said first sun gear and define said released mode of said second clutch.
 4. The transfer case of claim 3 wherein said third clutch includes a third clutch pack mounted between said housing and said second sun gear, and a third piston movable between a first position and a second position relative to said third clutch pack, said third piston is operable in its first position to engage said third clutch pack to brake rotation of said second sun gear and define said locked mode of said third clutch, and said third piston is operable in its second position to release said third clutch pack to permit rotation of said second sun gear and define said released mode of said third clutch.
 5. The transfer case of claim 4 wherein said first piston is disposed in a first pressure chamber, said second piston is disposed in a second pressure chamber, and said third piston is disposed in a third pressure chamber, and wherein said control system includes a source of hydraulic fluid, a first control valve in fluid communication with said first pressure chamber, a second control valve in fluid communication with said second pressure chamber, and a third control valve in fluid communication with said third pressure chamber, said control system further including a pump for supplying hydraulic fluid from said fluid source to each of said first, second and third control valves, and a controller for controlling actuation of said first, second and third control valves so as to control the supply of hydraulic fluid to and from each of said pressure chambers.
 6. The transfer case of claim 2 wherein said first clutch includes a drum housing fixed to said output shaft and a hub fixed to said carrier, wherein said first clutch pack includes a set of first clutch plates mounted to said drum housing which are alternately interleaved with a set of second clutch plates mounted to said hub, and wherein said biasing mechanism includes a pressure plate fixed for movement with said piston and which is engageable with said first clutch pack, and a spring acting between said drum housing and said pressure plate for urging said pressure plate into engagement with said first clutch pack.
 7. The transfer case of claim 3 wherein said second clutch includes a clutch drum fixed to said first sun gear such that said second clutch pack is mounted between said housing and said clutch drum.
 8. The transfer case of claim 1 wherein said ring gear meshes with said second pinion such that said first drive connection establishes a direct speed ratio with rotation of said input shaft causing said output shaft to be driven at a common speed, wherein said second drive connection establishes an underdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at a reduced speed, and wherein said third drive connection establishes an overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed.
 9. The transfer case of claim 1 wherein said ring gear meshes with said first pinion such that said first drive connection establishes a direct speed ratio with rotation of said input shaft causing said output shaft to be driven at a common speed, wherein said second drive connection establishes an overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed, and wherein said third drive connection establishes an underdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at a reduced speed.
 10. A transfer case comprising: a housing supporting an input shaft and an output shaft; a planetary gearset including a first sun gear, a second sun gear, a carrier coupled to said input shaft, a compound pinion gear supported on said carrier and having a first pinion meshed with said first sun gear and a second pinion fixed to said first pinion and meshed with said second sun gear, and a ring gear coupled to said output shaft and meshed with one of said first and second pinions; a first clutch operable in a locked mode to couple said ring gear for common rotation with said carrier and which is further operable in a released mode to permit relative rotation therebetween; a second clutch operable in a released mode to permit rotation of said first sun gear and which is further operable in a locked mode to brake said first sun gear; a third clutch operable in a released mode to permit rotation of said second sun gear and which is further operable in a locked mode to brake said second sun gear; and a control system for controlling actuation of said first, second and third clutches to establish three drive connections between said input shaft and said output shaft, wherein a first drive connection is established with said first clutch in its locked mode and each of said second and third clutches in its released mode, wherein a second drive connection is established with said second clutch in its locked mode and each of said first and third clutches in its released mode, and wherein a third drive connection is established with said third clutch in its locked mode and each of said first and second clutches in its released mode.
 11. The transfer case of claim 10 wherein said first clutch includes a first clutch pack mounted between said carrier and said ring gear, a piston movable between a first position and a second position relative to said first clutch pack, and a biasing mechanism for biasing said piston toward its first piston, said piston is operable in its first position to engage said first clutch pack to couple said ring gear for rotation with said carrier and define said locked mode of said first clutch, and said piston is operable in its second position to release said first clutch pack to permit relative rotation between said ring gear and said carrier and define said released mode of said first clutch.
 12. The transfer case of claim 11 wherein said second clutch includes a second clutch pack mounted between said housing and said first sun gear, and a second piston movable between a first position and a second position relative to said second clutch pack, said second piston is operable in its first position to engage said second clutch pack to brake rotation of said first sun gear and define said locked mode of said second clutch, and said second piston is operable in its second position to release said second clutch pack to permit rotation of said first sun gear and define said released mode of said second clutch.
 13. The transfer case of claim 12 wherein said third clutch includes a third clutch pack mounted between said housing and said second sun gear, and a third piston movable between a first position and a second position relative to said third clutch pack, said third piston is operable in its first position to engage said third clutch pack to brake rotation of said second sun gear and define said locked mode of said third clutch, and said third piston is operable in its second position to release said third clutch pack to permit rotation of said second sun gear and define said released mode of said third clutch.
 14. The transfer case of claim 13 wherein said first piston is disposed in a first pressure chamber, said second piston is disposed in a second pressure chamber, and said third piston is disposed in a third pressure chamber, and wherein said control system includes a source of hydraulic fluid, a first control valve in fluid communication with said first pressure chamber, a second control valve in fluid communication with said second pressure chamber, and a third control valve in fluid communication with said third pressure chamber, said control system further including a pump for supplying hydraulic fluid from said fluid source to each of said first, second and third control valves, and a controller for controlling actuation of said first, second and third control valves so as to control the supply of hydraulic fluid to and from each of said pressure chambers.
 15. The transfer case of claim 10 wherein said first drive connection establishes a direct speed ratio with rotation of said input shaft causing said output shaft to be driven at a common speed, wherein said second drive connection establishes a first overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed, and wherein said third drive connection establishes a second overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed.
 16. A transfer case comprising: a housing supporting an input shaft and an output shaft; a planetary gearset including a first sun gear, a second sun gear, a carrier coupled to said input shaft, a first pinion supported on said carrier and meshed with said first sun gear, a second pinion supported on said carrier and meshed with said second sun gear and said first pinion, and a ring gear coupled to said output shaft and meshed with one of said first and second pinions; a first clutch including a first clutch pack operably connected between said ring gear and said carrier and a first actuator movable between first and second positions relative to said clutch pack, said first actuator is operable in its first position to engage said first clutch pack for coupling said ring gear to said carrier, and said first actuator is operable in its second position to disengage said first clutch pack for permitting relative rotation between said ring gear and said carrier; a second clutch including a second clutch pack operably connected between said housing and said first sun gear and a second actuator movable between first and second positions relative to said second clutch pack, said second actuator is operable in its first position to engage said second clutch pack for braking rotation of said first sun gear, and said second actuator is operable in its second position to disengage said second clutch pack for permitting rotation of said first sun gear; a third clutch including a third clutch pack operably connected between said housing and said second sun gear and a third actuator movable between first and second positions, said third actuator is operable in its first position to engage said third clutch pack for braking rotation of said second sun gear, and said third actuator is operable in its second position to disengage said third clutch pack for permitting rotation of said second sun gear; and a control system for moving each of said first, second and third actuators between its first and second positions to establish three drive connections between said input shaft and said output shaft, wherein a first drive connection is established when said first actuator is in its first position and each of said second and third actuators is in its second position, wherein a second drive connection is established when said second actuator is in its first position and each of said first and third actuators is in its second position, and wherein a third drive connection is established when said third actuator is in its first position and each of said first and second actuators is in its second position.
 17. The transfer case of claim 16 wherein said first actuator includes a first piston slideably disposed in a first pressure chamber, said second actuator includes a second piston slideably disposed in a second pressure chamber, and said third actuator includes a third piston slideably disposed in a third pressure chamber, and wherein said control system is a hydraulic control system including: a fluid source; a pump; a first control valve operable in a first mode to supply high pressure fluid from said pump to said first pressure chamber and further operable in a second mode to vent fluid from said first pressure chamber to said fluid source; a second control valve operable in a first mode to supply high pressure fluid from said pump to said second pressure chamber and further operable in a second mode for venting fluid from said second pressure chamber to said fluid source; a third control valve operable in a first mode to supply high pressure fluid from said pump to said third pressure chamber and further operable in a second mode to vent fluid from said third pressure chamber to said fluid source; and a controller for controlling actuation of said control valves.
 18. The transfer case of claim 16 wherein said ring gear meshes with said second pinion such that said first drive connection establishes a direct speed ratio with rotation of said input shaft causing said outputs shaft to be driven at a common speed, wherein said second drive connection establishes an underdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at a reduced speed, and wherein said third drive connection establishes an overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed.
 19. The transfer case of claim 16 wherein said ring gear meshes with said first pinion such that said first driven connection establishes a direct speed ratio with rotation of said input shaft causing said output shaft to be driven at a common speed, wherein said second driven connection establishes an overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed, and wherein said third drive connection establishes an underdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at a reduced speed.
 20. A transfer case comprising: a housing supporting an input shaft and an output shaft; a planetary gearset including a first sun gear, a second sun gear, a carrier coupled to said input shaft, a compound pinion gear supported on said carrier and having a first pinion meshed with said first sun gear and a second pinion meshed with said second sun gear, and a ring gear coupled to said output shaft and meshed with said second pinion; a first clutch including a first clutch pack operably connected between said ring gear and said carrier and a first actuator movable between first and second positions relative to said clutch pack, said first actuator is operable in its first position to engage said first clutch pack for coupling said ring gear to said carrier, and said first actuator is operable in its second position to disengage said first clutch pack for permitting relative rotation between said ring gear and said carrier; a second clutch including a second clutch pack operably connected between said housing and said first sun gear and a second actuator movable between first and second positions relative to said second clutch pack, said second actuator is operable in its first position to engage said second clutch pack for braking rotation of said first sun gear, and said second actuator is operable in its second position to disengage said second clutch pack for permitting rotation of said first sun gear; a third clutch including a third clutch pack operably connected between said housing and said second sun gear and a third actuator movable between first and second positions, said third actuator is operable in its first position to engage said third clutch pack for braking rotation of said second sun gear, and said third actuator is operable in its second position to disengage said third clutch pack for permitting rotation of said second sun gear; and a control system for moving each of said first, second and third actuators between its first and second positions to establish three drive connections between said input shaft and said output shaft, wherein a first drive connection is established when said first actuator is in its first position and each of said second and third actuators is in its second position, wherein a second drive connection is established when said second actuator is in its first position and each of said first and third actuators is in its second position, and wherein a third drive connection is established when said third actuator is in its first position and each of said first and second actuators is in its second position.
 21. The transfer case of claim 20 wherein said first actuator includes a first piston slideably disposed in a first pressure chamber, said second actuator includes a second piston slideably disposed in a second pressure chamber, and said third actuator includes a third piston slideably disposed in a third pressure chamber, and wherein said control system is a hydraulic control system including: a fluid source; a pump; a first control valve operable in a first mode to supply high pressure fluid from said pump to said first pressure chamber and further operable in a second mode to vent fluid from said first pressure chamber to said fluid source; a second control valve operable in a first mode to supply high pressure fluid from said pump to said second pressure chamber and further operable in a second mode for venting fluid from said second pressure chamber to said fluid source; a third control valve operable in a first mode to supply high pressure fluid from said pump to said third pressure chamber and further operable in a second mode to vent fluid from said third pressure chamber to said fluid source; and a controller for controlling actuation of said control valves.
 22. The transfer case of claim 20 wherein said first drive connection establishes a direct speed ratio with rotation of said input shaft causing said output shaft to be driven at a common speed, wherein said second drive connection establishes a first overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed, and wherein said third drive connection establishes a second overdrive speed ratio with rotation of said input shaft causing said output shaft to be driven at an increased speed. 